METABOLISM 271 



abstracts heat from the body in being raised to body 

 temperature. 



The way in which the rate of metaboHsm rises as loss of 

 heat increases, with a fall of temperature, is shown by the 

 following results obtained by Rubner on a fasting dog : — 



„ ^ Eate of Metabolism In 



^^™P- ^- Cal. per k§. body wt. 



•65 68-5 



30 56-2 



25 54-2 



20 55-9 



15 630 



7 86-4 



In this chemical regulation energy-yielding materials, 

 usually carbohydrates and fats, are metabolised solely for the 

 purpose of heat production. 



Wind has a very marked effect upon the rate of cooling, 

 and consequently upon the rate of metabolism, especially in 

 an animal like the pig where the hair is scanty. Hill has 

 shown that in man exposure to a cold wind may nearly 

 double the energy expenditure. 



Critical Temperature. — In the case of the fasting dog 

 quoted above, the lowest rate of metabolism is reached in 

 an environment with a temperature of about 25° C. Below 

 this level the rate of metabolism is increased for heat pro- 

 duction by chemical regulation. As the temperature rises 

 above 25° C. not only is loss of heat decreased, but meta- 

 bolism is stimulated by the rise in temperature, so that heat 

 is produced beyond requirements, and its elimination is 

 hastened by physical regulation. The level of the external 

 temperature at which chemical regulation gives place to 

 physical, or vice versa, is known as the critical tempera- 

 ture. It is the temperature of minimum metabolism, and 

 consequently of lowest food requirements. 



Experimental evidence is lacking to determine the exact 

 critical temperature for domestic animals. It is probably 

 about 20° to 25° C. in the horse and pig, and somewhat 

 lower in rumiuants. In the individual it varies with the 

 condition of the coat and the amount of subcutaneous fat. 



The taking of food causes an increase in rate of metabolism. 



